Drilled shafts or piers are often used in the deep foundation industry because they provide an economical alternative to other types of deep foundations. Drilled piers are typically formed by excavating a cylindrical borehole in the ground and then placing reinforcing steel and fluid concrete in the borehole. The excavation may be assisted by the use of drilling fluids, casements or the like. When the concrete hardens, a structural pier suitable for load bearing results. These piers may be several feet in diameter and 50 feet or more deep. They are typically designed to support axial and tensile compressive loads.
Piles, usually made out of concrete, are generally used to form the foundations of buildings or other large structures. A pile can be considered a rigid or a flexible pile. The purpose of a pile foundation is to transfer and distribute load. Piles can be inserted or constructed by a wide variety of methods, including, but not limited to, impact driving, jacking, or other pushing, pressure (as in augercast piles) or impact injection, and poured in place, with and without various types of reinforcement, and in any combination. A wide range of pile types can be used depending on the soil type and structural requirements of a building or other large structure. Examples of pile types include wood, steel pipe piles, precast concrete piles, and cast-in-place concrete piles, also known as bored piles, augercast piles, or drilled shafts. Augercast piles are a common form of bored piles in which a hollow auger is drilled into the ground and then retracted with the aid of pressure-injected cementatious grout at the bottom end, so as to leave a roughly cylindrical column of grout in the ground, into which any required steel reinforcement is lowered. When the grout sets, the pile is complete. Piles may be parallel sided or tapered. Steel pipe piles can be driven into the ground. The steel pipe piles can then be filled with concrete or left unfilled. Precast concrete piles can be driven into the ground. Often, the precast concrete is prestressed to withstand driving and handling stresses. Cast-in-place concrete piles can be formed as shafts of concrete cast in thin shell pipes that have been driven into the ground. For the bored piles, a shaft can be bored into the ground and then filled with reinforcement and concrete. A casing can be inserted in the shaft before filling with concrete to form a cased pile. The bored piles, cased and uncased, and augercast, can be considered non-displacement piles.
A finished structural foundation element such as a pier or pile has an axial load bearing capacity that is conventionally characterized by components of end bearing (qb) and side bearing, which is a function of skin friction (fs). Loads applied at the top end of the element are transmitted to the sidewalls of the element and to the bottom of the element. The end bearing capacity is a measure of the maximum load that can be supported there, and it will depend on numerous factors including the diameter of the element and the composition of the geomaterial (soil, rock, etc.) at the bottom of the shaft. The side bearing capacity is a measure of the amount of load capable of being borne by the skin friction developed between the side of the pier/pile and the geomaterial. It depends on numerous factors, including the composition of the foundation element and the geomaterial forming the side of the element, which may vary with length (depth). The sum of the end bearing and side bearing capacities generally represents the total load that can be supported by the element without sinking or slippage, which could cause destructive movements for a finished building or bridge atop the foundation.
Although it is desirable to know the maximum end bearing and side bearing for a particular pier or driven pile, it is difficult to make such measurements with a high degree of confidence. Foundation engineering principles account for these difficulties by assigning end bearing and load bearing capacities to a foundation element based on its diameter and depth, the geomaterial at the end of the element and along its side, and other factors. A safety factor is then typically applied to the calculated end bearing and side bearing capacities. These safety factors are chosen to account for the large number of unknown factors that may adversely affect side bearing and end bearing, including geomaterial stress states and properties, borehole roughness generated by the drilling process, geomaterial degradation at the borehole-shaft interface during drilling, length of time the borehole remains open prior to the placement of concrete, residual effects of drilling fluids, borehole wall stresses produced by concrete placement, and other construction-related details. For example, it is common to apply a safety factor of 2 to the side bearing so as to reduce by half the amount calculated to be borne by skin friction. Likewise, a safety factor of 3 is often applied to the calculated end bearing capacity, reflecting the foregoing design uncertainties and others. Load Resistance Factor Design (LRFD) is an alternative analysis method used to design safe and efficient structural foundations by incorporating load and resistance factors based on the known variability of applied loads and material properties.
The use of safety factors, or LRFD factors, although judiciously accounting for many of the uncertainties in drilled shaft pier construction and driving piling, often results in such foundation elements being assigned safe load capacities that are too conservative. To compensate, builders construct larger, deeper, and/or more elements than are necessary to safely support a structural load, unnecessarily increasing the time, effort and expense of constructing a suitable foundation.
As a partial solution, it has been known to directly measure the end bearing capacity and skin friction of a drilled-shaft pier. This is typically accomplished at a production site by using one or more test piles.
Osterberg (U.S. Pat. Nos. 4,614,110 & 5,576,494) discloses a parallel-plate bellows placed in the bottom of the shaft before a concrete pier is poured. The bellows are pressured up with fluid communicated through a pipe coaxial with the pier. Skin friction is determined by measuring the vertical displacement of the pier (corresponding to the movement of the upper bellows plate) as a function of pressure in the bellows. Likewise, end bearing is determined by measuring pressure against the downward movement of the lower bellows plate, as indicated by a rod affixed thereto and extending above the surface through the fluid pipe. Upon completion of the load test, the bellows are depressurized. The bellows may then be abandoned or filled with cement grout, and in the latter case becomes in essence an extension of the lower end of the pier.
In that case, the non-functioning testing cell serves as the base of the pier and may thereby compromise the integrity of the shaft. In practice, a drilled shaft employing the “Osterberg cell” is often abandoned after testing in favor of nearby shafts that do not contain a non-functioning testing cell at their base. Because it is wasteful in terms of time, materials, effort and money to abandon a formed shaft merely because it was used for testing, there remains a need for a testing cell that causes less interference with use of the shaft after testing.